System Modelling (1)

Specification of individual requirements allows the construction ofvarious kinds of system models.

Diagrams are used to represent our picture of a system.

Different approaches can focus on:-

processes and the data effected by processes

data entities and the relationships between them

states a system can exhibit/show and the events that shift that system from one state to another

the sequence of events in all or part of a system (a “scenario”)

Dataflow analysis

Entity-relationship (E-R) analysis

State machine diagrams

Sequence diagrams + other diagrammatic representations used in UML

System Modelling (2) Dataflow analysis 1

A dataflow diagram (DFD) represents all or part of a system in an abstract (non-physical) way using just 4 symbols

Dataflow – data in motion. A “pipe” – unidrectional flow.

The dataflow arrow does NOT mean “Go To”

External Entity - external source or destination of system dataDatastore – data of the same type (an entity) Stationary data

or

A Process – an actvity that effects data or

System Modelling (3) Dataflow analysis 2

Dataflow

Dataflow contents e.g. amount_owing

External entity

Name of EE e.g. Supplier

Process

Process ID, e.g P1 or P2.1

Process name, e.g Verify_Pin

DatastoreDatastore ID, e.g D2

Datastore name e.g.Accounts

Dataflow analysis allows a system to be progressively “discovered” by “levelling”

System Modelling (4) Dataflow analysis 3

warehouseReorder System

Purchasing System

A context diagram puts the system in context in relation to related EE’s

High-level context diagram

System Modelling (5) Dataflow analysis 4

warehouse Process_stock_transaction

Purchasing System

inventory

P1

Produce_ReOrder_List

P2

ReOrders

D1

D2ReOrder_detail

Transaction_details

ReOrder_detail

exception_report

Level 0 DFD

System Modelling (6) Dataflow analysis 5

warehouse

Purchasing SystemProduce_

ReOrder_List

P2

inventoryD1

ReOrdersD2ReOrder_detail

trans_details

exception_report

Level 1 DFD of P1

Get_trans

P1.1 P1.2

Validate_trans

trans_details

transactionsD1

trans_details

valid_trans

stock_id,trans_qty

valid_flag,trans_qty

warehouse

reject_note

P1.3 P1.4

Update_inventory

stock_id,trans_qty

Create_reordervalid_trans

ReOrder_detail

Taking P1 to Level 1 has “exploded “ 1 process into 4 smaller ones

System Modelling (7) Dataflow analysis 6

Levelling should continue until an individualprocess need be decomposed no further – it is in a primitive form with probably no more than one dataflow output and whose logic could be described in a “short” piece of PDL

Note that if the top-level DFD had 8 processes and each of these processes was taken through 2 levels of decomposition, each yielding another 8 sub-processes, then our system would end-up with 512 primitive Processes (8 x 8 x 8)

There is no rule about how many levels are required but 2 to 3 levels is usually adequate and not all processes need to be taken down to the 3rd level.

(from Hawryszkiewycz, Systems Analysis & Design)

System Modelling (8) Entity-Relationship Modelling 1

Manager

Project

manages

[n]

1

Entity set

Entity set

Relationship set

Proj_Code

Proj_Name

Start_Date

Tot_Budget

Man_StartDate

Proj_Locn

Fin_Date. . .

RK1

RK2RK3

RK4

ABC

XYZ

IJKDEF

Loc4

Loc9

Loc5

Loc8

4000050000

70000

90000

1/10/10 30/4/11

7/8/10

1/1/11 5/7/11

9/3/11

8/9/118/9/10

Proj_Code

Man_No

Man_Budget

. . . RK13

5

5

3

RK2RK3

RK4

8000

15000

11000

32000

31/1/11

7/3/11

9/5/115/1/11

Man_No

Man_Name

Dept_ID

. . .

. . .

5

3 Merkel,A

Obama, B

FinHealth

. . .

. . .

Attributes

Separate instances (occurrences) of the entity

Separate instances (occurrences) of the relationship

Note that arelationship can have its own attributes

System Modelling (9) Entity-Relationship Modelling 2

Mapping a complete system using the E-R approach

(from Hawryszkiewycz, Systems Analysis & Design)

System Modelling (10) State Machine Models 1

A state machine diagram shows how a system responds to internal or external events.

It does not show the flow of data within the system

Often used for modelling real-time systems because these systems are often driven by stimuli from the system’s environment

Nodes represent system states

Arcs represent events that shift the system from one state to another

System Modelling (11) State Machine Models 2

A state machine diagram for a microwave oven

System Modelling (12) Sequence Diagrams 1

System Modelling (13) Sequence Diagrams 2

Several different types of diagrams are available within UML for representing different aspects of OO.

Sequence diagrams are dynamic models that show the sequenceof object interactions that take place.

A sequence diagram showing operations involved in collecting data from a weather station

The objects are:

:CommsController

:WeatherStation

:WeatherDataTime is represented vertically

Thin rectangle represents time when object is the controlling object in system

summarise (report)

System Modelling (14) Sequence Diagrams 3

1 Object that is an instance of CommsController receives a remote request to send a weather report. It acknowledges receiving this request.

2. This object sends a message to an object that is an instance of WeatherStation to create a weather report. The instance of CommsController suspends itself.

3 WeatherStation sends a message to WeatherData to summarise the weather data

4 Summary is computed and control returns to the WeatherStation object

5. The WeatherStation object sends a message to CommsController requesting it to transfer the data to the remote system. The WeatherStation object suspends itself

6. The CommsController object sends the report to the remote station, receives an acknowledgement, and then suspends itself waiting for the next request.